1. Field of the Invention
The present invention generally relates to electronic circuits and, more specifically, to terminals of contactless communication with distant elements of electromagnetic transponder type.
The present invention more specifically applies to short-range communication systems (with a range shorter than a few meters), for example Near Field Communication (NFC) systems.
2. Discussion of the Related Art
In near field communication systems for which the power consumption of the terminal is not a concern, said terminal periodically transmits (for example, every half second) an interrogation frame intended for transponders likely to be located in its field. When a transponder receives this frame, it responds thereto. The interrogation frame amounts to modulating a carrier according to a predefined coding. The carrier corresponds to an A.C. signal at the frequency on which the oscillating circuits of the terminal and of the transponder are tuned.
For systems in which the terminal power consumption is desired to be minimized, such interrogation frames are only transmitted when a transponder has been previously detected by the terminal. For this purpose, one uses the fact that the resonant circuit comprised by the transponder forms, even in the absence of a transmission, a load on the oscillating circuit of the terminal in the field of which it is located. This load variation in the presence of a transponder is then detected to consider that a transponder is present in the field and is capable of communicating with the terminal. Generally, the terminal periodically excites (for example, every half second) its oscillating circuit without modulating the carrier with data and detects a possible decrease in the voltage across the antenna with respect to a reference value. This enables decreasing the power consumption of the terminal.
However, for the detection to be reliable, the excitation frequency of the oscillating circuit must be accurate. The generation of such a frequency conventionally requires a quartz oscillator. Such an oscillator generates a relatively high consumption on each starting of the quartz oscillator.
Further, in the presence of a transponder having a resonant circuit which is not perfectly tuned, there is a risk of detection errors.